Closed-circuit re-breathers (CCRs) are used by divers, miners, firefighters and a variety of other personnel who must work under environmental conditions where breathable air is either unavailable or in short supply. Generally speaking, a CCR includes a carbon dioxide (CO2) scrubber that removes the CO2 produced by the person wearing the CCR. The CO2 scrubber includes one or more substances that will “scrub”, i.e. react with, the CO2 in order to remove the CO2 so that gas exiting the scrubber can be inhaled again by the person wearing the CCR. Since the removal of the CO2 is critical, it is important for the user to know when the CO2 scrubber is losing its ability to scrub the exhaled CO2.
A variety of approaches have been used to determine the scrubbing capacity that remains in a CCR that is in use. For example, U.S. Pat. No. 4,154,586 (Jones) discloses a method in which the CO2 scrubbing material changes color when it is spent. However, in underwater diving and fire-fighting applications, the user may not be able to see such a color change. Another approach is described in U.S. Pat. No. 4,146,887 (Magnante) where a temperature difference between the ambient environment and one location inside the scrubber is measured, and the measured temperature difference is used to predict and provide an “end-of-life” indication. However, variations in ambient conditions, e.g. temperature, can cause the end-of-life indication to come too early (the scrubber could continue to remove CO2) or too late (the scrubber ceases to remove enough CO2 before the indicated end-of-life) in the life of the scrubber.
Still another approach is described in U.S. Pat. No. 4,440,162 (Sewell) where temperature is measured at a predetermined location in the scrubber. When the temperature exceeds a pre-set value, an alarm is triggered. However, prior to the alarm, this approach does not provide the user with any way of knowing what the remaining capacity or utilized capacity of the CO2 scrubber is. In addition, the temperatures in the reactive material will depend on the ambient temperature, thus resulting in alarms being provided when an alarm should not be given.
Since the endurance of a CO2 scrubber varies with ambient temperature, ambient pressure and with a user's breathing rates, it is desirable to provide a user with updated capacity-information that has been generated by taking account of such operating parameters. However, the above-described prior art approaches are either impractical for certain applications, or do not provide such ongoing information.
Two known approaches exist that might give such desired ongoing information. U.S. Pat. No. 6,618,687 (Warkander) describes the use of temperature changes inside the space occupied by the CO2 reactive material to give nearly continuous readings for remaining capacity; and EU patent EP 1316 331 B1 (Parker) describes a method that compares temperature readings to pre-determined temperature distribution characteristics. Both compare the temperature at predetermined locations to the warmest part of the reactive material. Such a comparison achieves reasonably good end-of-life predictions when the highest temperature remains steady. Unfortunately, the highest temperature does not remain steady. For example, in FIG. 3 of this document, the highest temperature is somewhat steady for Time=15% to Time=55%, but then the highest temperature drops until the reactive material reaches the limit of its ability to remove a sufficient amount of CO2 (Time=100%).
Using the highest temperature to predict end-of-life may not be advisable for all types of scrubbers. For example, the method in Warkander and the method in Parker were developed with diving rebreathers, which tend to be less efficient than a rebreather for dry-land use. A low efficiency scrubber may last only half as long (i.e. 50%) as a high efficiency scrubber. In low efficiency scrubbers, CO2 will reach its level of exhaustion before the highest temperature starts to decline. For instance, had the recordings in FIG. 3 come from a low efficiency scrubber instead of a high efficiency scrubber (and only lasted half as long), the reactive material would have been deemed exhausted at time=50% instead of at time=100%. At time=50% the highest temperature (at T9) has still not peaked. In contrast, for high efficiency scrubbers, the highest temperature in the reactive material peaks before the reactive material is spent. Thus, the methods of determining the remaining capacity described in U.S. Pat. No. 6,618,687 and EP 1316 331 B1 will not work well for high efficiency scrubbers.
U.S. Pat. No. 7,987,849 (Heesch) describes a method for determining the consumption of a CO2 scrubber in a patient's respirator using measurements of the patient's breathing and comparing it to an estimate of the scrubber's maximum capacity of CO2 scrubbing. The maximum capacity of a CO2 scrubber may be known fairly well for a patient being breathed quietly in an operating room with a controlled ambient temperature. However, for a rebreather that is used where the conditions vary, the efficiency of a scrubber may vary from under 20% to over 80% of its maximum (theoretical) capacity (Nuckols et al., Life Support Systems Design; Simon and Schuster Custom Publishing, Needham Heights, M A 1996. ISBN 0-536-59616-6). Given this range of efficiencies, Heesch's method will not be accurate enough for many uses. For example, in underwater diving, the workload of the diver, ambient temperature range and ambient pressure range can vary significantly.
U.S. Patent Application 2014/0345610 (Unger) describes a method wherein a consumption indicator, consisting of a melting material, measures the total reaction heat, which is purported to be related to the consumption of reactive material. However, the temperature of the reactive material is, in practice, almost unaffected by the work rate (CO2 production) of the wearer. Therefore, such a consumption indicator will not work well in many situations.
U.S. Pat. No. 6,003,513 (Readey) describes a system that provides a general idea of the life of the reactive material based on where “localized heating” takes place. However, all of Readey's temperature sensors (shown as temperature strip 100 in Readey's FIG. 2) are placed in the flow of gas that is about to enter the reactive material. Readey's temperature sensors are not in contact with the canister or the reactive material. Therefore, they will read the temperature of the gas, but not the temperature of the reactive material. In addition, Readey's FIG. 8 shows that the temperature profile is assumed to have a local maximum that travels downstream as the reactive material is consumed. As is illustrated in the present FIG. 3 below, the temperatures in an actual scrubber do not show such a local maximum. Readey does not explain how the position of the local maximum relates to the CO2 level in the exhaust gas, the key end-of-life determinator. Therefore, Readey's method is inaccurate, and thus will not provide a reliable indication of remaining capacity and will not work well.
Since the endurance of a CO2 scrubber varies with ambient temperature, ambient pressure and with a user's breathing rates, and it is desirable to provide a user with updated capacity-information that has been generated by taking into account such operating parameters as to the remaining capacity or utilized capacity of the CO2 scrubber, which is something that the above-described prior art approaches do not do.